The ability to accurately detect vapor in a gas is important in many situations. For example, determining the concentration of flammable gases in a gas stream in combustion technology is important from both a safety and energy efficiency standpoint. In an organic chemical manufacturing facility, such as a refinery, for example, monitoring the concentration of vapors of certain volatile liquids in air is critical to the safety of the personnel in the area. In a home or consumer environment, the ability to automatically detect gas leakages is also critically important for protecting families and children, particularly in homes experiencing a leak in propane (LP) or natural gas lines or insufficient ventilation during work with solvents in a garage, next to an open-flame heating appliance.
Vapor sensors are particularly useful in automotive applications. As vehicle emission standards increase in stringency, engine control system designers must devise increasingly sophisticated strategies for the handling of fuel vapor generated by the evaporation of fuel contained within the tanks of the vehicle. Such fuel vapor is usually stored in one or more canisters. Each canister can be regenerated by atmospheric air flow through the canister. The resulting combined gas stream composed of air and fuel vapor is transferred into an air intake mechanism of the engine for combustion. If such regeneration of the canisters is not handled properly, the air/fuel ratio of the engine may be disturbed. This may create a problem because the tailpipe emissions of the engine or vehicle could very well increase if the resulting engine feed gas oxygen level falls outside an acceptable range.
Vapor sensors can also be utilized in fuel storage systems. Monitoring fuel storage tanks, particularly those underground, for hydrocarbon leaks is an exceedingly important environmental concern. Current detection/monitoring systems for monitoring leaks in fuel storage tanks can employ, for example, semiconductor, capacitive, and conductive liquid crystalline sensors or gas analyzers for detecting liquid or vapor leaks. These systems are complicated and very expensive.
A number of vapor sensor configurations and techniques have been attempted. Electrically conductive polymeric materials, such as conductive rubber, have been used for detecting liquid hydrocarbons, but must be placed at locations such as a sump where leaking liquid will collect and directly contact the sensor. Silicone polymer sensors have also been implemented. Silicone polymer sensors conduct electricity but are not affected by water. This type of sensor is sensitive to liquid hydrocarbon and possesses a low resistance, along with a high density of carbon black particles. This type of sensor is not responsive to gas-phase hydrocarbons.
Additional prior art sensors include combustion energy, flame ionization, gas chromatography, chemical, differential thermal analysis and optical sensors. These types of sensors are all generally expensive, complicated and are not suitable for the fabrication of detectors of flammable vapors, particular those involving hydrocarbon leaks. Still other vapor sensors are affordable and sensitive and operate using metal-oxide sensing elements but are unacceptably slow and unstable for unsupervised use over long periods of time. Thus, they are impractical and unsafe to implement.
Thus, the fast, reliable, sensitive, low-power, low-false-alarm, ambient-condition-independent and affordable detection of non-specific, flammable vapors in homes, garages, or industry presents a significant technical challenge. By way of summary of the above, prior art sensors to date can be classified generally in three categories. The first category of prior art sensors includes sensors which are low-cost and sensitive but unreliable and slow. Examples of such sensors include metal oxide or swelling-polymer type sensors. The second category of prior art sensors includes sensors which are sensitive and fast but expensive. Such sensors include IR-optical absorption type sensors. The third category of prior art sensors includes sensors which are sensitive, moderately fast and low-cost but which are unreliable and too specific to be of any use to broad sensing applications. Such sensors include electrochemical and Pellistor-type sensors, both of which are well known in the sensor arts.
Based on the foregoing, the present inventors have concluded that a need exists for a reliable and efficient flammable vapor sensor with broad applicability to a wide range of consumer and industrial situations and products. The present inventors thus are disclosing herein a solution to the aforementioned problems, which is based on a technique for sensing the presence of air-borne organic fuel vapors on the basis of their ability to change the bulk thermal conductivity properties of air and by processing this measurable change to provide an approximately universal (i.e., versus temperature and fuel type) alarm set-point relative to an LEL (Lower Explosive Limit) for any number of flammable gases or vapors.